\input pstfig
$
\def\myR{2.2}
\thinline
\pscircle(0,0){\myR}
\thickline
\psset{labelsep=0.2ex}
\pnode(\myR; 135){A}
\pnode(\myR; 45 ){B}
\pnode(\myR; -30){C}
\pnode(\myR; -80){D}
\pcline(A)(B)
\Bput{a}
\pcline(B)(C)
\Bput{d}
\pcline(C)(D)
\Bput{c}
\pcline(D)(A)
\Bput{b}
$
\bye
